A plant is broadly defined as any member of the group of living organisms which, although characterized by irritability or excitability, is typically lacking in locomotor movement or rapid motor response. Plants may be divided into two classes: vascular and non-vascular plants. Non-vascular plants, e.g., bryophytes and algae, do not have vascular tissue which enables the circulation of nutrients and sap within the organism. This class of plants is not within the subject matter of this invention, and will not be discussed further herein.
Vascular plants are plants which have evolved physiologically complex conducting systems, and the efficiency of these vascular systems has led to tremendous ecological diversification. These plants range from grasses to large trees, and provide vegetables, fruit, flowers, wood and various photosynthates as products of plant growth and reproduction.
Vascular tissue is comprised principally of xylem and phloem. Xylem is the principal water and mineral-conducting tissue in vascular plants, and is a complex tissue composed of non-living, lignified tracheids, vessels and fibers and their associated living parenchyma cells. Xylem also may provide mechanical support, especially in plants with secondary xylem, i.e., wood. Xylem, while thus providing some mechanical support, functions primarily to convey water from the roots to the aerial portions of the plant.
Phloem tissue is that part of the vascular system which transports the elaborated food materials (photosynthates) from the leaves. The phloem consists typically of sieve tubes and companion cells. Transportation of the phloem is typically from leaves to roots.
Xylem tissue forms vascular paths along the long axis of the plant stems from roots to the tips of the aerial, i.e., above-ground, portions of the plant. From these longitudinal pathways extend radial xylem tissue, termed xylem rays, which conduct water and water nutrient solutions to the leaves, shoots and other surface portions of the plant. The phloem tissue forms a distinct pathway which surrounds the xylem.
The stems, that is, the above-ground portions of vascular plants, vary from plants less than 1.2 cm in height to forest giants towering 100 meters or more. Nonetheless, all stems have essentially similar vascular systems. The growing tip of any young stem has cells which are initially very similar, but which differentiate to form the plant subsystems as the stem grows. In most dicotyledonous plants, the cells which are nearest the center of the stem become xylem cells and those towards the circumference of the stem become phloem cells. In most monocotyledons, the vascular tissues occur in the form of separate small bundles with the phloem surrounding the xylem tissue.
Generally, fruit-bearing and other vascular plants are grown by placing a seed, sprout or other natural plant portion in fertile soil or hydroponic medium and allowing roots to form and extend to provide anchorage, and absorption of water and minerals which are conducted by the vascular system to the aerial portions of the plant which, in turn, provide the desired plant products.
It will be appreciated that traditional techniques for the propagation of plants consume substantial amounts of time and acreage. Many plants propagated by man for the production of fiber, wood, photosynthates, blossoms or fruits require long maturation cycles to afford requisite physical and nutritive support (i.e., roots) for the shoots which are the desired plant product.
Current techniques focus excessive resources toward optimizing growth for the entire plant when, in fact, the shoots' metabolic maximization is often the ultimate objective. Parenthetically, the term shoot is used herein to indicate any aerial stem of a vascular plant rather than the commonly-employed reference to a young growing branch or twig with leaves.
Attempts have been made to propagate plants in an artificial environment through the use of hydroponic techniques. In this soil-free culture method plants are grown with their roots immersed in a solution containing the necessary mineral salts, or rooted in a sand or vermiculite medium moistened with such a solution. This method, while providing certain advantages, is not totally efficient due to the necessity to support the entire plant when only minimal anchorage is supplied by the roots, and the fact that the nutrient solutions must be drained periodically to avoid oxygen starvation or dilute exuded toxins. Also, to the extent the growth solutions in hydroponic systems come in contact with ambient air and the bacteria therein, contamination of the growth solutions is an ongoing problem. Further, hydroponic methods have inherent difficulties because plants which have evolved to be rooted in a nutrient soil are required to root in an unnatural medium. For these reasons, most plants which are hydroponically grown are of the smaller, rapidly-maturing variety and large-scale hydroponic production of fruits and vegetables has not been economically feasible.
Workers in the plant sciences have developed techniques for forming plants from individual cells or tissues by placing shoot explants on a solid or liquid media in culture vessels. However, these techniques are applied to the regeneration of whole plants, that is, the propagation of plant cells or tissue to form roots and precursors of the aerial plant portions (i.e., roots and shoots) which are then further grown to form complete plants in either natural soil or under hydroponic conditions. During the early development of cultured plants, a high ambient humidity is required to protect the growing shoot, and as the shoot matures a gradual decrease to normal ambient humidity is required to progressively harden the plant prior to exposing the plant to natural growth conditions. Accordingly, it has been the practice to sequentially transfer the tissue culture, or to sequentially modify the humidity in a closed greenhouse to meet the requirements of the growing plant. Thus, the existing practice in applied botanical-horticultural science is to propagate and harden a whole plantlet or sapling, which is then deposited at a location where further maturation of the whole plant takes place. Accordingly, extended maturation periods, inflexible crop rotation, restrictive growing zones and requisite soil conditions significantly limit crop production.
Further, such micropropagation has posed unique problems for many tissues such as woody fruits, tropicals and ornamental plants due to the need to modulate or change nutrients over time. Multiple transfers of propagating tissues from one growth medium to another as the plant matures have thus been required to prepare the plant for growth in a natural environment.
While the in vitro propagation of plants has provided advantages, especially in the regeneration of plants from protoplast cells, many potential advantages of culture propagation have not been realized in practice due to the difficulty of culturing plant cells or tissues to mature plants. In fact, many species are resistant to micropropagation techniques. While I do not wish to be bound to any particular theory, this failure may be due to the fact that the growth media comprise an artificial interface for plant growth which fails to adequately support the developing plant.